Q: What would you experience if you were going the speed of light?

The original question was: If I’m moving at the speed of light towards you and I throw a tennis ball at you (at, say, 5 m/s), what do you observe? Will I hit you first, the tennis ball, or both at the same time, or will something else happen entirely? The problem that I’m having thinking about this is that if the tennis ball did hit you first, then it would have been moving faster than the speed of light (relative to you). On the other hand, if it were moving at the speed of light relative to you, then it was moving at the same speed as me relative to you, thus both myself and the tennis ball will hit you simultaneously. If this is the case, however, then the tennis ball would have been moving at a speed of 0 m/s with respect to me.


Mathematician: First of all, let me point out that you will never travel at the speed of light (see this for details). It would take an infinite amount of energy to get anything with mass (e.g. you or your mama) going at that speed. Burning all the oil (and plants, and animals) on our planet and converting them into kinetic energy would get you going really fast, but would give you exactly 0% of the total energy that you would actually need to get going that speed (since any number divided by infinity is zero). But that doesn’t mean that it wouldn’t be fun to speculate about what would happen if you were going at light speed.

Due to a relativistic effect known as time dilation, the faster that you move with respect to some object O, the more time slows down for the object O (from your perspective). This isn’t just an issue of you seeing clocks attached to O tick slowly, they actually DO tick slowly from your vantage point (no experiment you could possibly do would conclude otherwise). There is symmetry though. You moving past O at 10,000 miles per hour is indistinguishable (as far as the laws of physics are concerned) from O moving past you at 10,000 miles per hour. That means, from the perspective of a person strapped to O, clocks attached to you are ticking slowly (i.e. your time is slowed down). As you approach the speed of light (with respect to O), this time dilation effect becomes more and more pronounced. When you are exactly AT the speed of light (impossible, but bare with me) no time whatsoever will elapse for O (i.e. a clock strapped to O will stop ticking completely) from your perspective. The upshot of this is that you’ll get wherever you are going without witnessing any time pass for anything not moving along with you. One reason this is really trippy is because if we view light coming from a distance object (such as a far away sun), from our perspective it might have taken years to get from us. But from the perspective of the photon (i.e. the light particle) no time will have elapsed on the journey! Yes, true physics is even weirder than crazy person made up physics.

A possibly even wackier effect crops up as the result of length contraction (another consequence of relativity). If you move towards object O at a fast speed you will notice that O will be compressed (i.e. shrunk) along the direction of your motion. So if O is a hippo, and you are going fast enough, it will look like one damn flat hippo.

Um, something like this I guess?

As you approach the speed of light, this effect becomes increasingly pronounced, and at at the speed of light itself O will have zero length in the direction you are traveling. In particular, if you are on a straight race track, and traveling at the speed of light, the race track will be compressed to zero length so that the starting line and the finishing line will be on top of each other. The race will be over as soon as it begins.

Another consequence of light speed travel is that you’d become the most dangerous thing imaginable (move over, Chuck). Since your mass is positive, infinite speed implies that your momentum is infinite. Hence, if you crashed into anything (and you would…after all, from your perspective the universe is flatter than a pancake in the direction you’re heading) it would get hurtled at insane speeds (since it would absorb some of your momentum). Of course, you’d also be dead pretty much instantly as you collided with object after object (each traveling at the speed of light with respect to you). And no, armor wouldn’t help.

Okay, so now to address the original question. What would happen if while traveling at the speed of light towards me you attempted to throw a ball at me? The answer is that you would have no time to actually do the throwing, because from your perspective you would run into me instantly. At which point, if I had any ninja skills, I would probably break those out. If you were traveling at near the speed of light (with respect to me), but not quite at it, and then threw the ball at 5 m/s (with respect to you) in my direction, the velocities would not simply add like you would expect based on Newtonian mechanics. Instead, you’d have to apply relativistic velocity addition which is a bit more complicated. In particular, the speed of the ball with respect to me will be less than the sum of your speed and 5 m/s. At low speeds this effect is not noticeable (speeds are additive to very close approximation), but at speeds close to the speed of light the effect becomes very pronounced.

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23 Responses to Q: What would you experience if you were going the speed of light?

  1. david newman md says:

    you mentioned length contraction as velocity increases toward the speed of light. is there any connection between a flat race track and a flat universe (two dimesional)?

  2. The Physicist Physicist says:

    Nope.
    Also, are you talking about horse race tracks or what?

  3. david newman md says:

    race tracks and everything.. but how can anyone walk around in a flat universe boy, talk about low clearance! like in being john malkovitch whee the office they worked in was five feet high

  4. I understand (or don’t I) that as you approach the speed of light objects become so massive that no energy would be capable of moving them, is that right?

    Then, why we don´t see this effect in reality as photons travel at the speed of light?

    Are photons real? Light could not be transmitted by waves like sound because in space there is no “medium” for the waves to travel?

    Thanks

  5. Bo Brymer says:

    A blur of energy, color and light

  6. John says:

    I just wanted to ask this one question because I feel like I understand most ‘simple’ things that have been posted around the internet about special relativity but there is one thing bugging me:
    If a body is travelling very close to the speed of light and the time on board for them has slowed, what would a person on the outside of them — looking to where they would be — see?
    Would they see anything? Or would they see them in a slow-motion like way because of time dilation? I’m confused about how a person on the outside would see them or whether they would be able to see them at all because they’re travelling at the speed of light after all. Please talk in a completely realistic way. Could a person actually see anything? Or would you have to presume that the person had been squished in the direction of travel?

    Sorry for the amount of question, but I hope you get the idea.

    Thanks, John.

  7. Jnaneshwara G K says:

    If a object travel with a speed of light, then it looks like a rays where you cannot recognize the real object travelling. ex. If a superman or spiderman travelling with speed of light you will not recognize who they are? so you need a superfast camera to detect them in that speed, most probable…900000000000M fixel camera or more powerfull? So If such fast object hits a earth or having a big mass what may be result of it? so how we can detect a speeding photol or electron ?

    Thanks
    Jnaneshwara G K

  8. The Physicist The Physicist says:

    You let it hit something. Trying to take a picture isn’t worth the bother.

  9. Jonathan says:

    The thing is, if you were traveling at the speed of light wouldn’t you not be able to move forward? Meaning the signals in your brain would not be able to move. This would also apply to signals in a computer chip. And if you can’t move forward, you can only move backward, so technically wouldn’t you be dead?. Also, if you were sitting on something (such as a chair), you would vibrate into it.

  10. David says:

    As far as effects go. The simple act of moving through air particles at a rate of speed not even vaguely near the speed of light, the friction would obliterate you and possible cause a nasty combustion/force wave in the surrounding area.
    I suspect that at that immense of a speed, air would almost act as a solid in terms of friction.

  11. Ismael says:

    In regards this I thought your mass would increase and theoretically once you have more mass than object around you, gravity on you increases, thereby you draw other object from around you towards you.

    If and hopefully not you cannot support said mass, your gravity would create a black hole in you, imploding you almost instantly.

    Hence unless you can have infinite energy to counteract your infinite mass, you die.

    Also as David mentioned, and always bothered me though your description of space getting squeezed managed to confuse me more/ clarify a bit, does this mean things like air have no effect on said body as I thought you would experience some resistance of some sorts which would lead to friction and at that speed should also mean death?

  12. Justin Kyllo says:

    As you approach the speed of light your mass is infinite and your dimension approaches zero, meaning when/if you collided with anything it would simply be just protons/neutrons, but with your unlimited mass vs their super small mass it would be like a fly hitting you.

  13. ED says:

    What I don’t understand is a person travelling close to the speed of light, does his biological clock slow down? How does he stay younger on his return?

  14. Glenn Shrom says:

    If I am traveling at 0.9 c, and I observe object O accelerate from 0.5 c to 0.8 c, am I seeing the opposite happen for O from what we usually say happens as something approaches the speed of light? Am I seeing the mass of O decrease as it accelerates, time speeding up for O, etc.? When we normally talking about something approaching the speed of light, we normally talk about it from the perspective of an observer that is traveling slower than that object. But would the same changes be observed if we are going faster than it while it accelerates?

  15. The Physicist The Physicist says:

    @Glenn
    The object starts out moving at 0.4c* and it ends up moving at 0.1c*. So, the relativistic effects decrease. Someone with a different perspective will describe the situation differently (but consistently). After all: relativistic effects are relative.

    *There are some subtleties involving how speeds are combined in relativity, so someone moving at 0.9c seeing something moving at 0.8c won’t see it moving at 0.1c, but the idea is the important thing here.

  16. Paradigm says:

    I’m very confused. This (http://science.howstuffworks.com/science-vs-myth/what-if/what-if-faster-than-speed-of-light1.htm) article says that time dilation makes time move slower for objects moving very fast. eg. A clock on a space ship would advance less than a clock on a planet it passes by. Therefor if you looked out the window at the people below they would be walking around in fast motion as thier clocks speed ahead of yours. Also to them you would be moving in slow motion in your spaceship. The faster you travel the slower your time passes and the slower you can move. At the speed of light you would be frozen completely and forever.

    We know that stronger gravitation slows down time, so it fits in well that infinate mass at the speed of light could contribute to the time freeze.

    But you seem to be saying that time on the planet below actually slows down relative to you, and for people on the planet looking into your spaceship, your time also slows down to them. So is that other article wrong or am I missing something? Is it different if the two objects have vastly different mass, like a ship vs a planet?

    Suppose nothing exists in the universe except for two similar spaceships with crews and a clock, and both sets sum up to the same mass. You’re telling me that if one ship moves really fast towards the other one, then they are both moving. I can understand that.
    But then you say that if one crew looks at the other crew’s clock they will see it going slower than theirs, and the same happens for the other crew. So when they meet and compair clocks who’s clock will be ahead or behind?

    We know from experements that clocks on moving vehicles move slower. (http://www.alternativephysics.org/book/TimeDilationExperiments.htm) Therefore I think that the time in the reference frame of the moving object is slowed while time outiside of that reference frame is relatively sped up.

    I think that in the case of the two ships, the one that is pushed gains more mass, gravity, energy, etc which maybe cause its time to slow down relative to the other ship which although apparantly moving relatively at the same speed, gains no energy, and therefore should really be considered as non-moving. Besides, the non-moving ship is also motionless relative to the vacume/darkmatter where as the pushed ship is moving relative to the vacume, darkmatter, and the other ship.

  17. The Physicist The Physicist says:

    @Paradigm
    It’s deeply weird, but the situation you’re describing (where both ships see the other ship’s clocks running slow) is exactly the case.
    There’s an old post here that tries to explain that, and another here that talks about the twin paradox which is related, but very different.

  18. Ed says:

    What I would like to know is what happens to us biologically to stay younger when we travel near the speed of light? Do our cells slow down too?

  19. Matt says:

    @Ed, I believe that it’s less of anything happening to us biologically and more everything around you happening much quicker. From reading through this, if one were to hit the speed of light then time would basically stop for that person. So from that state of time being stopped, while we would age relative to Earth time (or whatever non-lightspeed object you wish to compare it to) our bodies would not actually be doing anything since time would have essentially stopped for us.

    This is a layman’s view so correct me if I’m wrong.

  20. Ed says:

    Thank you Matt for the answer. However, after returning from near speed of light journey to Earth, there is an age difference between the traveler and the observer on Earth. If there is an age difference, is there a biological difference too? If yes, how?

  21. zeke says:

    So, I’ve been reading, and I understand that there is not enough energy for me to travel at the speed of light, but is there enough energy in the universe for the universe to zip past me at the speed of light? …or is that WHY it would take infinite energy for me to go that fast?

  22. michael says:

    @Ed
    Time would pass normally for you but everyone else would be moving much faster and age quicker. It doesn’t “keep you younger longer” in the way you might want it to… It doesn’t make time FEEL slower for you. It just makes time pass much faster for everyone else.

  23. West says:

    One question…if a photon experiences no time, then is any given photon everywhere at once with respect to itself? For example, if a photon from a light source, like a star, traveled to a mirror one light year away, bounced off and then traveled to another mirror another light year away, how could it experience both mirrors simultaneously?
    I know it “traveled” only in respect to us observing it…but if a photon is everywhere at once, and arrives instantaneously, then aren’t all photons everywhere?

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